Traditional optical transmission systems have primarily employed a conventional intensity modulation format, i.e. on-off keying (OOK), due to its simplicity at both transmitter and receiver. Recently advanced coherent modulation formats such as differential phase shift keying (DPSK), differential quadrature phase shift keying and m-array phase shift keying [1-4] have attracted increased attention. In coherent communications networks, data is encoded into the phase instead of intensity of the optical signals, providing numerous advantages over the traditional OOK format, including robustness, better tolerance to nonlinearity and crosstalk, increased receiver sensitivity and spectral efficiency [1]. Due to these advantages, many research laboratories have exploited advanced modulation formats for ultra-high bit-rate and long-haul transmission systems [5-8].
Despite the robustness of coherent optical systems, reliable optical performance monitoring (OPM) [9, 10] is still a critical part of network infrastructure. In particular, for quality of service assurance and optimal network performance. Conventionally, DPSK signals are demodulated and detected for performance monitoring by a Mach-Zehnder (MZ) delay interferometer and a high-speed balanced receiver, respectively [1], adding a significant cost and complexity to the network.
Several relatively simple OPM techniques have been reported to monitor impairments of phase modulated optical signals. These include single impairment monitoring methods for group velocity dispersion (GVD) [11] and optical signal-to-noise ratio (OSNR) monitoring [12] using the radio-frequency (RF) power spectra, and amplitude and phase Q-factor measurement using asynchronous amplitude and phase histograms [13]. Moreover, multi-impairment monitoring schemes, including GVD and OSNR monitoring using asynchronous amplitude histogram evaluation [14]. GVD and first-order polarization mode dispersion (PMD) monitoring using asynchronous amplitude histogram evaluation [15] or asynchronous delay tap sampling [16] have been established. However, these conventional electro-optic based monitoring schemes reply on high-speed detectors, thus their typical operating bandwidth is limited to about 100 GHz and costs remain relatively high.
Besides the above methods, a variety of all-optical OPM schemes that work for high speed phase-encoded optical signals have been presented. These include OSNR monitoring using two-photon-absorption in a semiconductor micro-cavity [17] and an optical delay interferometer [18]. Alternatively all-optical signal processing based on nonlinear optics is considered as a method to overcome the limitations imposed by the electronic bandwidth. Several monitoring methods, including GVD monitoring using cross-phase modulation (XPM) in a highly nonlinear fiber (HNLF) [20] have been presented. Although impressive results have been obtained, these techniques do not offer multi-impairment monitoring functionality which is essential for next generation optical communication networks.